This invention relates to hydraulic control mechanisms, and more particularly, to such control mechanisms including a pulse-width-modulated valve and an electrical switch. Specifically, this invention relates to control mechanisms for isolating the pressure oscillations of a pulse-width-modulated valve from the switch.
Electronic controls are being utilized in many power transmissions to control the actuation of fluid operated friction devices, such as clutches and brakes. In a number of the electronically controlled transmissions, the fluid pressure levels in the various fluid operated friction devices are controlled by pulse-width-modulated solenoid valves. These solenoid valves operate at a substantially constant A/C frequency in the range of 60 to 70 Hz. By changing the duty cycle by controlling the percentage of time that the solenoid is in the "on" state vs. the percentage of the time that the solenoid is in the "off" state, the apply pressure of the fluid operated devices can be modulated at any desired pressure level between zero and a maximum system pressure. These control systems have been found to be cost effective in controlling fluid operated friction devices, however, they do present a side effect of inducing high frequency noise or hydraulic pressure pulsations into the fluid circuit.
These control devices also utilize pressure operated electrical switches which provide various diagnostic and control functions. It is preferable to use a low cost pressure operated electrical switches. However, low cost switches do not have structural advantages which will prevent the pressure pulsations from acting on the switch. Thus, the switches undergo significantly more off/on signals than are necessary during normal transmission operation.